Global positioning receivers calculate their position by precisely timing the signals sent by GPS satellites. Each GPS satellite transmits a message which includes the time of message transmission and the satellite's position at the time of message transmission. By differencing the given time of message transmission from the time of message receipt, GPS receivers can determine the transit time of each satellite message. By multiplying the transit time by the speed of light, the GPS receiver can calculate an approximate distance from the receiver antenna to the transmitting satellite, also known as a pseudorange. Taking the pseudoranges calculated from several satellites together with the given satellite positions at the time of message transmission, a GPS receiver can estimate its current position and clock offset.
Thus, accurate GPS positioning relies upon receiving clear, direct signals from the GPS satellites. Unfortunately, GPS signals can bounce or reflect off objects prior to being received by a receiver antenna. Such reflected signals travel a longer distance than unreflected signals, resulting in a longer transit time and therefore distorting positional calculations. This effect is known as the multipath effect and it causes significant positional calculation error. One form of multipath error occurs when a GPS receiver antenna observes a multipath signal from a satellite even though the direct sightline is obstructed. Knowledge of whether a sightline to a GPS satellite is obstructed can help detect possible multipath error and improve the accuracy of positional calculations.